In order to enhance conductivity of a π-conjugated polymer, it is necessary to dope the polymer with a dopant. However, the polymer having extended π-conjugation has high planarity in the polymer chain, and thus the tendency of the crystallization or stacking of the polymer chains is high because of the affinity of the π-bonds. In addition, doping of the π-conjugated polymer with a dopant enhances the planarity and the affinity by the π-conjugation, and thus enhances the tendency of the stacking. Therefore, it was a difficult problem to enhance the solubility (by heat or solvent) of the π-conjugated polymer while enhancing the electric conductivity.
Here, there has been proposed a polymer in which an alkyl group, an alkoxy group, or the like is introduced on the side chain of a π-conjugated polymer (Patent Document 1). However, doping is necessary to enhance the electric conductivity to ten to the negative fifth power (s·m) or less, at which the polymer can be called a conductor. The doping enhances the planarity and the π-conjugation affinity, and thus there occurs a problem that sufficient solubility to a solvent cannot be achieved.
In view of the use of the conductive polymer and in view of the easiness of the handling, it is desirable that the polymer is soluble to a solvent or meltable by heat, and in addition a self-supported film or self-supported molded body with sufficient electric conductivity after molding or film formation can be obtained. Conventionally, when these conductive polymers are used, a polymer film is formed on a substrate by direct electropolymerization or vapor exposure to provide conductivity to the substrate, or thin film polymerization is performed by immersing the substrate in a solution of an oxidizing agent and a conductive polymer precursor monomer, followed by heating and so on. Thereafter, the obtained polymer film is processed by doping and so on.
However, the substrate needs to be semiconductor or conductor to perform the electropolymerization, and the corrosion resistance to the electrolyte is also required, and therefore, the choice of the substrate is limited. Furthermore, the thin film polymerization by direct vapor requires that an oxidizing agent exists uniformly on a thin film which is a polymerization field, and is not satisfactory in terms of the film formation control. In a polymer capacitor application, microasperity is formed on the surface in order to increases the surface area, and it is difficult to form a conductive polymer on a sufficiently uniform surface.
There has been proposed several methods of dissolving a conductive polymer in an organic solvent. Patent Document 2 discloses a method of manufacturing poly (3,4-di-substituted thiophene) by use of 3,4-di-substituted thiophene, an inorganic ferric salt and an oxidizing agent. Patent Document 3 discloses a water dispersible powder including a polymer T mainly having repeating thiophene units and at least one other polyanion polymer P. However, the method in Patent Document 2 is a method of obtaining powder or a method of performing oxidative polymerization directly on the target surface, and thus it is impossible to dissolve the polymer obtained by the method in a solvent or water. Furthermore, the product obtained by the method in Patent Document 3 is just a water dispersible dispersoid, and is not soluble to an organic solvent at the molecular level
Furthermore, various researches have been made for more direct method of nanodispersing to a solvent. Patent Document 4 disclose a method of providing a microdispersoid solution at nano level by pulverizing polyaniline, which is essentially insoluble to a solvent, to nano level, and co-dispersing, to a solvent, the pulverized polyaniline with a dispersant of a sulfonic acid anion emulsifier such as SDS (dodecylbenzenesulfonic acid) or PTS (para-toluenesulfonic acid) having high affinity to polyaniline and a solvent. However, the polyaniline is not substantially dissolved in the solvent, and thus the surface of the coating film is uneven, and it is not possible to form a self-supported film (also referred to as a homogeneous film. This mean a film formed by itself and without forming pinholes), and therefore it is not possible to form a film after coating unless combined with a binder.
Furthermore, Patent Document 5 discloses a polythiophene solution containing, in water or a mixed solvent of water and an organic solvent miscible to water, polythiophene having a molecular weight of 2,000 to 500,000 and obtained by oxidation chemical polymerization in the present of a polyanion of a polystyrene sulfonic acid, and a polyanion having a molecular weight of 2,000 to 500,000 and derived from a polystyrene sulfonic acid.
The Patent Document proposes a method of manufacturing poly(ethylenedioxide-substituted thiophene) (PEDOT) which is soluble or dispersible to water or an alcohol solvent by use of oxidative polymerization in the present of polystyrene sulfonic acid (PSS) and an oxidizing agent. However, the obtained PEDOT/PSS is dispersed in water, but not completely dissolved, and thus it is difficult to suppress partial stacking between PEDOTs, and therefore dissolution of the conductive polymer was insufficient. The reason is as follows. A sulfonic acid group acts as a dopant group or a water-soluble functional group, but does not substantially have affinity or compatibility to the monomer. There is no functional group compatible with an alkoxyl group in order to control localization of an alkoxyl group and suppress the planarity of the π-conjugated polymer, which is important for the case where aniline, thiophene, pyrrole, or like an aromatic or a heterocyclic compound having an alkoxyl group is used as monomer component.
Furthermore, Patent Document 6 disclose a method of forming an organic solution by performing oxidative polymerization of aniline or aniline derivative in a solvent containing an organic acid or inorganic acid in the present of a strongly hydrophobic anionic surfactant, followed by precipitation, isolation, and purification, and thereafter performing extraction by use of an organic solvent immiscible to water.
However, the emulsifier used in the Patent Document is a low-molecular sulfonic acid-based, and aniline is hydrochlorinated before polymerization, thereafter the aniline salt is subjected to salt substitution by use of a sulfonic acid-based emulsifier. However, in reality, sufficient salt exchange is hard to occur, and therefore polyanion obtained by the synthetic method of the Patent Document is insoluble to a solvent, and thus only solvent dispersion in a micro dispersing state is obtained, which is problematic.
In addition, the sulfonic acid-based emulsifier is used in an equivalent molar amount to aniline, and thus 50% or more of the emulsifier remains unused for doping, and the unused emulsifier needs to be removed upon use. Therefore, the washing process is complicated, which is problematic. Furthermore, it is very difficult to design the low-molecular emulsifier in a way to introduce a function to enhance solubility to a solution, and a function to suppress the stacking of polyaniline. Even though polyaniline is dissolved temporarily in a solvent, micro-agglomeration due to the stacking (crystallization of PANI) occurs shortly, which is problematic. Furthermore, the present inventors conducted the re-test and confirmed that according to the method of the Patent Document, the localization of an alkoxyl group in the aniline derivative having an alkoxyl group, and chemical and structural relaxation of affinity at quinoimide binding position and amino binding position of the polyaniline are insufficient, and therefore the stacking is not suppressed.
Furthermore, Patent Document 7 discloses a method of emulsifying a solution obtained by dissolving, in water or an organic solvent, (A) a monomer having a sulfonic acid functional group and a radical polymerizable functional group, and (B) a monomer of aniline or the derivatives thereof; introducing, in the monomer (B), a sulfonic acid structure derived from the monomer (A), and thereafter polymerize the monomers (A) and (B) in the present of a polymerization initiator to obtain a conductive polymer with the polymer of (B) and the polymer of (B) intertwined.
However, in the method of the Patent Document, because ammonium persulfate salt is used as a water-soluble oxidizing agent and a radical initiator, it is difficult in reality to obtain the ideal mesh structure of the vinyl-based polymer and polyaniline as mentioned in the specification. Therefore, according to the method of the Patent Document, there's a problem in reality that a substantial amount of vinyl polymer not containing PANI exists, and a dope monomer not incorporated in the vinyl polymer exists in PANI, and thus the obtained product is very nonuniform and unstable.
For example, Patent Document 8 discloses a conductive polyaniline composition containing (a) a protonated and substituted or unsubstituted polyaniline complex and (b) a compound having a phenolic hydroxyl group, which is dissolved in an organic solvent substantially immiscible to water.
However, the conditions for the solvent effective in the Patent Document cannot be applied to a solvent somewhat miscible to water. In addition, the applicable monomer needs to be highly oil-soluble or needs to have a highly oil-soluble alkyl group. Therefore, for a monomer having a hydrophilic substituent such as an alkoxyl group, sufficient function cannot be expected under the conditions of the polymerization solvent and the dopant compound representing a sulfonic acid group.
By the way, the conductive polymer composition can be used for a counter electrode for a dye-sensitized solar cell and an antistatic film. Patent Document 10 discloses a counter electrode for a dye-sensitized solar cell having a conductive polymer layer on a plastic film provided with a transparent conductive layer.
In the Patent Document, the conductive polymer layer is formed by applying a dispersion containing a conductive polymer, and removing the solvent. However, because the conductive polymer is a layer in which fine particles are dispersed, and thus the adherence to the transparent conductive layer is low, and thus it is necessary to enhance the surface energy of the transparent conductive layer by plasma treatment or the like. In addition, polystyrene sulfonic acid is used as a dispersant in Examples of the Patent Document. In this case, there exists a free sulfonic acid which does not contribute to doping of the conductive polymer, and thus the solvent becomes an aqueous solution. Therefore, when the solution is applied on a film substrate, the range of choice of the solvent and the film substrate surface is very broad, and thus pinholes due to nonuniformity of the conductive polymer coating film easily occur. In addition, because the polarity of the coating film is high due to the remaining sulfonic acid group, the coating film has low durability to acetonitrile and an ionic liquid which are commonly used as an electrolyte solution, and therefore peeling of the coating film easily occurs. Because of such causes, there occurs a problem that a transparent conductive film is corroded by iodine in the electrolyte. Therefore, the conductive polymer layer has a problem in the long-tem stability as a counter electrode, and thus is insufficient to replace a platinum counter electrode with.
Furthermore, Patent Document 11 discloses an antistatic film obtained by applying antistatic material containing a polythiophene compound, acidic polymer and sugar alcohol on a thermoplastic resin film.
However, in the Patent Document, the antistatic material contains a sugar alcohol as an essential component, and thus the transparency and antistatic property of the obtained antistatic film is good. However, because only acidic polymer such as polystyrene sulfonic acid is used as a doping agent to the polythiophene compound, the antistatic film continuously absorbs moisture, and therefore the adherence and the antistatic property can deteriorate, which is problematic.